The proposed research will explore new approaches to the application of quantitative electron microscope (EM) autoradiography for studies in cell biology. These new approaches will include: tract autoradiography for use with high energy isotopes; EM autoradiography with the scanning electron microscope; and test specimens for soluble compound autoradiography. As new procedures are developed, they will be applied to specifc cell biology problems dealing with: cell surface molecules such as receptors, ion channels and pumps; internalization of membrane components; and intracellular pathways for transport and secretion.